ISDN connection keeps freezing!

I am running XP (all updates and SP1), and ISDN. At different times during connection - 1 - 20 hours, the conneciton itself freezes - the computer thinks it is connected, the icon is still displayed in the task bar saying the same, but no internet responses. If I open up the Connection dialog and click disconnect, that dialog freezes and the disconnect button gets stuck. If I end and restart explorer and try to reconnect, it freezes the start bar., so I have to reset each time this happens. It is not the modem because I have tried with an external USB and with an internal. I also experience this problem after a fresh install, so I have no idea how to resolve this or where the issue lies.. if anybody can shed some light I'd really appreciate it.. its sending my phone bills thru the roof since there is a once off charge each time I connect...